Metroidvania is a fan-made term for the gameplay style introduced into the Castlevania video game series with the release of Castlevania: Symphony of the Night for the original Playstation.This name is a combination of the words Metroid and Castlevania.
The Metroidvania gameplay style features one large contiguous map for the player to explore, as opposed to the linear series of separate levels presented in Castlevania titles prior to Symphony of the Night. Player progress through the story is directed by obstacles that require specific power-ups to pass.
The name arose because this system of using one unified world for a side-scrolling action game was first introduced by Nintendo in Metroid for the NES.
Enough factual writeup. Although I've never played a pre-Symphony Castlevania title, i can't imagine being restricted to a gauntlet of rooms after having so much freedom. A big part of the atmosphere comes from feeling utterly surrounded by the castle and its denizens. The only link you ever get to freedom, if any, is a small courtyard before the castle's entrance at the game's beginning. This concept shows how imitation in games should be done: Don't steal the entire game. Take an existing concept and portray it in an interesting way.